* Ethanol output falls 36,000 bpd to 821,000 bpd * Stocks fall 761,000 to 19.53 million barrels By Michael Hirtzer CHICAGO, July 11 (Reuters) - U.S. ethanol output plunged to its lowest level in nearly two years last week as soaring corn prices pushed many biofuel refineries into the red, government data showed on Wednesday. At least three U.S. ethanol plants -- two in Nebraska and one in Indiana -- are idling as the highest corn prices in more than a year squeeze margins in the worst spell since a string of bankruptcies in 2007 and 2008. Ethanol production last week fell 4 percent, or 36,000 barrels per day to 821,000 bpd, the lowest since the week ending July 23, 2010, according to the Energy Information Administration. Stocks of the biofuel fell 761,000 barrels to 19.53 million barrels, the lowest since January. "It's high corn prices and not-so-high ethanol prices," said Wally Tyner, an energy economist at Purdue University. "If we continue to have drought and corn continues to go up, there will be others that suspend production." Ethanol plants are losing money, with the average margin in Illinois at a negative 32 cents per gallon of fuel produced, according to Reuters data. And margins could get worse, with the existing corn stockpile dwindling and the conditions of the developing crop the worst in 24 years due to sizzling temperatures and drought conditions in the U.S. Midwest. The U.S. Agriculture Department reduced its estimate of U.S. corn production in a monthly supply and demand report early Wednesday. The government, which had initially projected the largest corn crop ever, pegged the crop at 12.970 billion bushels, the biggest since the record-large crop of 2009. The USDA also cut its estimate of the corn used in U.S. ethanol production by 100 million bushels in the 2012/13 grain marketing year ending Aug. 31, 2013. But Agriculture Secretary Tom Vilsack said that the government has no plans to change its Renewable Fuel Standard that this year sets a target for 13.2 billion gallons of ethanol this year.